The invention relates to the packing of polysilicon, especially of chunk polycrystalline silicon (chunk polysilicon).
Polysilicon is deposited from trichlorosilane by means of the Siemens process, for example, and then comminuted, ideally without contamination. A method for automatic crushing and a corresponding apparatus are described in EP 1 645 333 A1.
For applications in the semiconductor and solar industries, chunk polysilicon with minimum contamination is desired. Therefore, the material should also be packed with low contamination before being transported to the customer.
Typically, chunk polysilicon for the electronics industry is packed in 5 kg bags with a weight tolerance of +/−F 50 g. For the solar industry, chunk polysilicon in bags containing a weight of 10 kg with a weight tolerance of +/−max. 100 g are customary.
Tubular bag machines suitable in principle for packing chunk silicon are commercially available. A corresponding packing machine is described, for example, in DE 36 40 520 A1.
Chunk polysilicon is a sharp-edged, non-free-flowing bulk material having a weight of the individual Si chunks of up to 2500 g. Therefore, in the course of packing, it has to be ensured that the material does not penetrate the customary plastic bags in the course of filling, or in the worst case even completely destroy them. In order to prevent this, the commercial packing machines have to be suitably modified for the purpose of packing polysilicon.
With commercial packing machines, it is generally impossible to comply with the purity demands which are made on chunk polysilicon, since the composite films typically used, because of the chemical additions, can lead to increased contamination of the chunk polysilicon.
From EP 1 334 907 B1 is an apparatus for the cost-effective, fully automatic transporting, weighing, portioning, filling and packing of high-purity polysilicon fragments, comprising a conveyor channel for the polysilicon fragments, a weighing device of the polysilicon fragments, which is connected to a hopper, deflection plates made from silicon, a filling device, which forms a plastic bag from a highly pure plastic film, comprising a deionizer which prevents static charging and therefore contamination of the plastic film with particles, a welding device for the plastic bag filled with polysilicon fragments, a flowbox which is fitted above the conveyor channel, weighing device, filling device and welding device, and which prevents contamination of the polysilicon fragments by particles, a conveyor belt having a magnetically inductive detector for the welded plastic bag filled with polysilicon fragments, wherein all the components which come into contact with the polysilicon fragments are sheathed with silicon or clad with a highly wear-resistant plastic.
DE 10 2007 027 110 A1 describes a method for packaging polycrystalline silicon, comprising filling polycrystalline silicon by a filling device into a freely suspended, completely formed bag, and then closing the bag thus filled, wherein the bag consists of high-purity plastic with a wall thickness of from 10 to 1000 μm. Preferably, the closed plastic bag filled with polycrystalline silicon is introduced into a further plastic bag of PE with a wall thickness of from 10 to 1000 μm, and this second plastic bag is closed. Here, the first bag is thus inserted into a second bag (double bag).
In principle, there are two forms of bags, namely flat bags and free-standing bags.
Tubular films can be obtained by means of an extruder. Extruders are known as conveying units which, according to the principle of function of the screw conveyor, express solid to viscous and extrudable materials under high pressure and high temperature homogeneously out of a shaping orifice. This process is referred to as extrusion. Thermoplastic polymers such as PVC, PE, PP are known as extrudable materials.
Bags can be manufactured from the tubular films thus obtained, by sealing and subsequent division from the tubular film. Such bags are also commercially available in ready-made form.
Experience has shown that bags made from commercial PE films having undefined properties, which are filled with chunk polysilicon, are damaged in the course of transport. The sharp-edged chunk silicon causes damage, for example in the form of penetration and torn-open weld seams.
This problem gave rise to the objective of the invention.